Polarizers used in polarizing plates are optical devices for polarizing natural light or arbitrarily polarized light in a particular direction, and are widely used in display devices such as liquid crystal display devices or organic light emitting diode (OLED) devices. Currently, polyvinyl alcohol-based polarizing films, containing an iodine-based compound or a dichroic dye, in which molecular chains are oriented in a predetermined direction, are generally used as the polarizers used in the display devices.
The polyvinyl alcohol-based polarizing film is manufactured by a method of dyeing a polyvinyl alcohol-based film with iodine or a dichroic dye, stretching the polyvinyl alcohol-based film in a predetermined direction, and performing a cross-linking process, and in this case, the stretching process may be performed by a wet stretching process using a solution such as an aqueous boric acid solution or an aqueous iodine solution, or a dry stretching process performed in the atmosphere, and the like, and the stretching magnification is generally 3 times or more.
However, in the manufacturing process in the related art, in order to perform the stretching without causing any break, a polyvinyl alcohol-based film before being stretched is required to have a thickness of more than 60 μm. This is because when the polyvinyl alcohol-based film has a thickness of 60 μm or less before being stretched, the swelling degree of the polyvinyl alcohol-based film is increased, and the breaking strength deteriorates due to the thin thickness, and thus, breaking may be easily generated in the stretching process.
Meanwhile, in accordance with the recent trend for slimness of display devices, polarizing plates are also required to have a thinner thickness. However, when a polyvinyl alcohol-based film having a thickness of 60 μm before being stretched is used as in the related art, there is limitation in decreasing the thickness of a polarizer. Therefore, studies for manufacturing a polarizer having a thinner thickness have been attempted.
Korean Patent Application Laid-Open No. 2010-0071998 discloses a method for manufacturing a thin polarizing plate by using a laminate manufactured by coating a hydrophilic polymer layer onto a base material layer or co-extruding a base material layer forming material and a hydrophilic polymer layer forming material. However, in the case of the coating or the co-extrusion method, the polyvinyl alcohol layer and the base material layer are not easily separated after being stretched, and high peel-off strength is required to separate the polyvinyl alcohol layer and the base material layer, so that in the separation process, there occurs a problem in that a polyvinyl alcohol layer is damaged or deformed, and the like, and as a result, there is a problem in that optical properties such as the polarization degree of the polyvinyl alcohol film deteriorate.
Further, when the coating method or the co-extrusion method is used, a polarizing plate is manufactured by a system of melting a polyvinyl alcohol resin, and then extruding the resin, or preparing the resin into a coating solution, and then performing the application of the coating solution, so that physical properties of a polyvinyl alcohol film manufactured are easily changed according to the extrusion conditions, the coating conditions, or the film-forming conditions, and thus physical properties of a polyvinyl alcohol finally prepared deteriorate, and it is also difficult to implement uniform physical properties.